This invention relates to voltage reference circuits and, in particular, to bandgap voltage reference circuits which are compensated for power supply voltage variations.
Bandgap voltage reference circuits are well known and widely used in the art and typically provide an output voltage that is independent of temperature. The output voltage has substantially zero temperature coefficient and is produced by summing together two voltages such that one of the voltages has a positive temperature coefficient while other has a negative temperature coefficient.
In general, the positive temperature coefficient is produced by using first and second transistors operating at different current densities such that the first transistor is operating at a lower current density than the second transistor. By connecting a resistor in series with the emitter of the first transistor and then coupling the base of the first transistor and the other end of the resistor across the base and emitter of the second transistor, a delta V.sub.BE voltage across the resistor is produced that has a positive temperature coefficient. This positive temperature coefficient voltage is combined in series with the V.sub.BE voltage of a third transistor which has a negative temperature coefficient such that a composite output voltage having a very low or zero temperature coefficient is provided. This third transistor typically has its base coupled to collector of the first transistor, an emitter coupled to a first supply voltage terminal and a collector coupled to a second supply voltage terminal through a load resistor. Further, this third transistor is responsible for absorbing the change in current which is caused by power supply variation. Therefore, as a power supply increases, the current through the third transistor increases, thereby resulting in a increased V.sub.BE voltage across the third transistor. This increased V.sub.BE voltage of the third transistor then causes the output voltage of the bandgap circuit to increase. Therefore, it can be said that a positive slope function exists for the output voltage of the bandgap circuit as a function of power supply variation and, in general, the output voltage of a bandgap circuit is not independent of power supply variations.
One improvement that prior art has made to the bandgap circuit in order to reduce its output voltage dependence with respect to power supply variations utilizes a shunt PNP transistor and is fully described in U.S. Pat. No. 3,617,859, entitled "Electrical Regulator Apparatus Including A Zero Temperature Coefficient Voltage Reference Circuit", by Robert C. Dobkin and assigned to National Semiconductor Corporation. However, though this solution may work theoretically, it is not practical since typically a PNP transistor requires a much larger area than an NPN transistor and is typically much more difficult to fabricate.
Another improvement that prior art has made to the bandgap circuit in order to reduce its output voltage dependence with respect to power supply variations is fully described in U.S. Pat. No. 4,628,248, entitled "NPN Bandgap Voltage Generator", by Mark S. Birrittella et al and assigned to Motorola Inc. This describes an all NPN transistor approach to compensate for power supply variations which has the negative result of substantially increasing the number of components and size of the voltage reference circuit.
Yet another improvement that prior art has made to the bandgap circuit in order to reduce its output dependence with respect to power supply variation is disclosed in IEEE International Solid State Circuit Conference on Thursday, Feb. 16, 1989 on pages 120-121 by Texas Instruments, Inc. in Dallas, Tex. However, the output voltage of the bandgap circuit does vary with temperature as is clearly shown in FIG. 4 of the article. Therefore, some compensation for power supply variations has been attained, but at the cost of a loss of temperature compensation.
Still another improvement that prior art has made to the bandgap voltage reference circuit in order to reduce its output voltage dependence with respect to power supply variations is disclosed in IEEE Journal of Solid State Circuits, Volume 23, No. 5, Oct. 19, 1988, entitled "A 4-Ns 4K.times.1-Bit two-Port BiCMOS SRAM" by Yang. The circuit shown in FIG. 12 does provide some compensation for power supply variations, however, as stated in the article, simulations show that the reference output voltage changes about six millivolts for a one volt change in the power supply. For many applications, this change in output voltage may be too large and unacceptable.
Hence, a need exists for a voltage reference circuit having an output voltage that is independent of temperature and power supply variations.